The present invention relates to devices for generating foam for use in fire fighting and specifically to a foam generator which provides for automatic balancing of pressure differentials between incoming pressurized water and pressurized air.
Foam generators utilizing pressurized water and pressurized air in combination with a surfactant are useful in fire fighting. There are certain well known means of mixing air, water, and a surfactant to generate foam, including mixing chambers, venturis, and nozzles.
U.S. Pat. No. 4,981,178 issued to Bundy on Jan. 1, 1991 discloses an apparatus for generating fire-fighting foam using a mixing chamber.
U.S. Pat. No. 4,505,431 issued to Huffman on March 19, 1985 for "Apparatus for Discharging Three Commingled Fluids" and U.S. Pat. No. 4,474,680 issued to Kroll on October 2, 1984 for "Foam Generating Apparatus and Method" disclose venturi-type foam generators.
It has been very difficult in the past to produce a simple device for generating foam from the mixing of pressurized air and pressurized water. (The foam also requires the presence of a soap or surfactant which is introduced into the water prior to the foam generator.) Pressure balancing between incoming pressurized air and incoming pressurized water requires elaborate measures to control both the air volume and pressure and the water volume and pressure. It has generally been necessary to use very complicated devices to balance the volumes and pressures or to require the operator to manually adjust the volumes and pressures on a continuous basis during operation to maintain a balance. Thus a skilled operator is typically required to operate such systems.
If a balanced pressure is not maintained, the quality of the foam being generated can be affected. Various types of foam may be desirable for particular applications. In some situations a dry foam is desirable; in other situations, a wetter foam is desirable. Too much water or too much air can result in a foam that is not efficient for the intended purpose. For example, in some situations, the most desirable type of foam contains sufficient moisture to aid in smothering a fire while it is sufficiently dry to cling to surfaces. If a balanced pressure and volume of water and air is not maintained, the result can be a foam that is either too wet or too dry or that has other deficiencies with respect to the desired quality. The volume of water in relation to the volume of air determines the consistency of the generated foam, so the control of both pressure and volume is necessary to assure the desired foam quality.
The prior art emphasizes the importance of maintaining balanced pressures between the water and air supplies. Bundy, at column 3, beginning at line 12, discusses the problem of achieving the proper combination of air pressure and volume with water pressure and volume to achieve the desired quality of foam. Bundy also discusses the desirability of maintaining equal pressure in the air and water supplies.
The prior art has addressed the problem of balancing the air and water supply pressure in a foam generator by various expedients as mentioned above. Even with the fairly complex and expensive means employed, the operation of a foam generating apparatus for fire fighting requires the services of an experienced operator and even then much experimentation is necessary. For example, even the simple act of changing a hose attached to the apparatus often requires difficult and time consuming rebalancing of the system.
It has been suggested that a high degree of turbulence may contribute to the quality of foam produced in that a finer foam structure is obtained. Foam comprised of large bubbles is less useful for typical fire-fighting applications. It may therefore be desirable to both balance the pressures of the incoming water and air and do so in a way that maximizes turbulence.
Prior foam generating systems lack a means to automatically cut off the flow of water and air into the system's hose when the hose nozzle is turned off. This may create an unsafe condition if system air and water pressures are not precisely balanced. If the system water pressure exceeds the system air pressure, closing the nozzle may cause a "slug" of water to build inside the hose. When the operator again opens the nozzle and expects a relatively low-density foam to emerge, the slug of water that squirts forth may cause the operator to lose control of the hose. Conversely, if the system air pressure exceeds the system water pressure, a pocket of air may build in the hose when the nozzle is closed. Subsequent opening of the nozzle may send forth a burst of oxygen onto a flame thereby aiding the spread of a fire rather than extinguishing it. Either a slug of water or burst of air may thus result in serious injury to the hose operator or bystanders. Given the difficulty in prior art foam generating systems of maintaining a precise balance between system air and water pressure, it has been difficult to prevent these unsafe conditions.
The problems and limitations of the prior art are overcome by the present invention as summarized below.